Indirectly measuring optical power in a fiber laser system, i.e. without putting a power measuring device directly into the optical path, is desirable, because of a lack of accessibility to the measuring point and the high cost of power measuring equipment. In order to perform an indirect power measurement, sampling of the optical power in the fiber being measured is required. Sampling of optical power propagating in the core of an optical fiber is commonly done using a fused fiber tap coupler or by detecting the scattered light from a fiber fusion splice location. Conventional sampling methods often work reasonably well with low power single-mode (SM) fiber laser systems, but have limitations in fiber optical systems with large-mode-area (LMA) or multi-mode (MM) fibers and high-power fiber laser systems. One drawback of conventional methods with LMA or MM fibers is that the sampling ratio is not identical across all transversal modes of the fiber. Thus, detected signal levels from both fused fiber tap couplers and splice scattering light monitors not only varies with the power propagating in the fiber core, but also fluctuates with the composition of the transversal modes. Moreover, in high-power fiber laser systems, a fuse fiber tap coupler or splice point is not always feasible or desirable, because they might not be able to handle the optical power level or because of the associated penalties on the output power and beam quality.
Prior art references, including U.S. Pat. No. 5,015,067 issued May 14, 1991 to Lavatter; U.S. Pat. No. 5,319,195 issued Jun. 7, 1994 to Jones et al; U.S. Pat. No. 7,146,073 issued Dec. 5, 2006 to Wan; and U.S. Pat. No. 7,957,438 issued Jun. 7, 2011 to Simons et al, disclose the use of a photodetector in close proximity to an optical fiber for measuring optical power. However, the Lavatter reference discloses using fluorescent light from a special coating to detect cladding light. Jones et al discloses detecting cladding light by out-coupling some of the cladding light. The Wan reference discloses using a special cladding treatment to scatter cladding light and monitoring the cladding light power, and Simons et al disclose using the light, which failed to couple into the fiber core, to estimate the power inside the core.
An object of the present invention is to overcome the shortcomings of the prior art by providing an optical power monitor that only utilizes Rayleigh scattering from the core of an optical fiber.